The present invention relates to a directional coupler system and, more particularly to a balance network directional coupler system.
Testing of communication lines, such as telephone lines and network cables is a vital function for operation of communication systems. Typically, testing involves determining the operational status of a communication line. For instance, for a twisted pair telephone line, operational status would include the length of the twisted pair line, the number and location of any taps or splices on the line, and the level of attenuation that the line imposes upon a communication signal traveling across the line.
To determine operational status of a communication line, test equipment is typically placed on both ends of the line. In the case of a twisted pair telephone line, one end is generally located at a central office site, and the other end is usually located either at a residential or commercial establishment. Prior art dual-ended testing systems and methods that use testing equipment on both ends of a communication line are inherently more costly and logistically complex to implement than a single-ended testing system that requires testing equipment on only one end of a communication line. However, prior art systems have focused on dual-ended testing rather than single-ended testing because of difficulties imposed by single-ended testing.
Many of the difficulties of single-ended testing relate to the loss of signal quality due to its travel in two directions along a communication line. Another problem with single-ended testing involves transmission signals interfering with reception signals. In single-ended testing, both a transmitter and a receiver are located at the same end of a communication line. As a result, a first signal transmitted from a first end may be reflected back to the first end. If a second signal is transmitted near the time that the first signal is received at the first end, interference with reception of the first signal may result.
Prior art attempts to reduce this type of interference has resulted in devices commonly referred to as directional couplers. In general, a directional coupler isolates transmitted signals sent on a communication line at one end of the line from signals received at the same end of the communication line. A directional coupler generally includes a transmission port that is coupled to a signal transmitter that generates signals to be transmitted on to the communication line. The transmission port also includes an amplifier section which amplifies the signals to be transmitted. A directional coupler also generally includes a communication line port that is coupled to the communication line being tested to both pass the signals to be transmitted on to the communication line and to receive reflected signals. The communication line port also generally includes transformers to assist in passing the signals to be transmitted on to the communication line. A directional coupler further includes a receiver port that is coupled to processing devices that process the reflected signals received at the communication line port of the directional coupler.
Prior art directional couplers have only been partially successful in isolating transmitted signals from received signals. This partial success has been due to several factors, including port designs that unduly alternate test signals and burdensome requirements for multiple transformers. This has resulted in less than desirable quality for various tests of communication lines and added weight and bulk of testers.
A balance network directional coupler in accordance with the invention allows for effective coupling to sources of transmitted signals, communication lines, and devices that process received signals. The balance network directional coupler includes a transmission port, first and second amplifiers, a reception port, a communication line port, a transformer system, and a balance network. The first amplifier has a first gain of one polarity, the first amplifier also has an output terminal and an input terminal electrically coupled to the transmission port. The second amplifier has a second gain of a polarity opposite the polarity of the polarity of the first gain. The second amplifier also has an output terminal, and an input terminal electrically coupled to the transmission port.
The transformer system has first and second windings each having first and second terminals, the first and second terminals of the first winding being electrically coupled to the communication line port, the first terminal of the second winding being electrically coupled to the reception port, and the second terminal of the second winding being electrically coupled to the output terminal of the second amplifier. The second winding of the transformer system has an equivalent impedance at the first and second terminals based on an impedance of the transformer system and an impedance of a communication line.
The balance network has first and second terminals, the first terminal of the balance network electrically being electrically coupled to the output terminal of the first amplifier, the second terminal of the balance network being electrically coupled to the reception port. The balance network is configured to have an impedance relative to the equivalent impedance at the second winding of the transformer system selected so that the reception port is substantially decoupled from the transmission port. In a further aspect the ratio between the magnitude of the first gain to the magnitude of the second gain is substantially equal to the ratio of the impedance of the balance network to the equivalent impedance at the second winding of the transformer.